1. Field of the Invention
The present invention relates to a driving device for driving an end-luminescent line head provided with end-luminescent electroluminescent elements (EL elements) arranged in an array.
2. Description of the Related Art
A recently developed electrophotographic line printer is one of electrophotographic printers. The electrophotographic line printer comprises a photoconductive drum, a charger, a line head, a developing unit and a transfer unit, which are arranged closely around the photoconductive drum. The luminescent elements of the line head project light signals corresponding to image signals on the circumference of the photoconductive drum charged by the charger to form an electrostatic latent image on the circumference of the photoconductive drum, the developing unit applies toner to the circumference of the photoconductive drum to develop the electrostatic latent image in a toner image, and then, the transfer unit transfers the toner image to a recording sheet. Trials have been made to use end-luminescent EL elements as luminescent elements for the line head.
As shown in FIG. 6, an end-luminescent EL element 56 is formed, for example, by sandwiching an active layer 51 of a zinc sulfide containing an active element between dielectric layers 52 and 53, and forming flat electrodes 54 and 55 respectively on the outer surfaces of the dielectric layers 52 and 53. When a voltage is applied across the flat electrodes 54 and 55 of the end-luminescent EL element 56, the active layer 51 emits flat light beams through its end surfaces. As shown by way of example in FIG. 7, an end-luminescent line head 58 comprises a substrate 57, a plurality of linear end-luminescent EL elements 56 formed on the substrate 57 in a parallel arrangement by a thin-film forming process, and a rod lens array, not shown, disposed opposite to the end surfaces of the end-luminescent EL elements.
FIG. 8 shows a known driving circuit for driving the end-luminescent line head 58 provided with m.times.n end-luminescent EL elements 56. The driving circuit has m common electrodes and n channel electrodes for the matrix driving of the m.times.n end-luminescent EL elements 5. 6A power supply 59 applies a supply voltage of 24 V to a high-voltage unit 60. Then, the high-voltage unit 60 applies a positive voltage V.sub.P and a negative voltage V.sub.N to a common driver 61 having a plurality of transistor circuits, not shown, for alternately applying the voltages V.sub.P and V.sub.N to the common electrodes of the end-luminescent line head 58. A channel driver included in a controller 62 applies a driving voltage to the channel electrodes of the end-luminescent line head 58. The channel driver is provided with a plurality of transistor circuits for selectively applying an input voltage to the channel electrodes of the end-luminescent line head 58 according to print signals. Each end-luminescent EL element 56 of the end-luminescent line head 58 emits light when the voltages applied to the common electrode and the channel electrode connected thereto exceed a threshold voltage. The controller 62 controls the respective transistor circuits of the common driver 61 and the channel driver for synchronous operation to drive the end-luminescent EL elements 56 of the end-luminescent line head 58 selectively for light emission.
The power supply 59 applies a voltage of +5 V as a logic voltage to the controller 62, i.e., a control circuit.
When the power supply 59 of the driving circuit shown in FIG. 8 is disconnected from the circuits, the outputs of the high-voltage unit 60 drops to a ground level (GND). Then, the output V.sub.P of the high-voltage unit 60 starts falling to GND and the output V.sub.N of the high-voltage unit 60 starts rising to GND with a delay after the output voltage of 24 V of the power supply 59 has started falling to GND as shown in FIG. 9. Whereas the high voltages V.sub.P and V.sub.N are applied continuously to the common driver 61 during this delay, application of the logic voltage of 5 V to the control circuit is interrupted immediately after the power supply 59 has been turned off. Consequently, the transistor circuits of the common driver 61 become unstable and there is the possibility that the common driver 61 and the end-luminescent line head 58 are destructed.